A sandcastle only lasts until the next tide comes in. To build a sand castle the sand must be moist enough for the grains to stick together. So it has to be near the water. But that puts the castle in position to be buried by the next high tide. Oh well. You can build another one when the tide goes back out!

Tides

Tides
are daily changes in the level of ocean water. They occur all around the globe.
High tides
occur when the water reaches its highest level in a day.
Low tides
occur when the water reaches its lowest level in a day. Tides keep cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two high tides and two low tides approximately every 24 hours.

Below, you can see the difference between high and low tides (
Figure
below
). This is called the
tidal range
.

Where is the intertidal zone in this picture?

Why Tides Occur

The figure below shows why tides occur (
Figure
below
). The main cause of tides is the pull of the Moon’s gravity on Earth. The pull is greatest on whatever is closest to the Moon. Although the gravity pulls the land, only the water can move. As a result:

Water on the side of Earth facing the Moon is pulled hardest by the Moon’s gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide.

Earth itself is pulled harder by the Moon’s gravity than is the ocean on the side of Earth opposite the Moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.

With water bulging on two sides of Earth, there’s less water left in between. This creates low tides on the other two sides of the planet.

High and low tides are due mainly to the pull of the Moon’s gravity.

Spring Tides and Neap Tides

The Sun’s gravity also pulls on Earth and its oceans. The Sun is much larger than the Moon, so is its pull greater than the Moon’s? The pull of the Sun’s gravity is much less because the Sun is much farther away. The Sun’s gravity strengthens or weakens the Moon’s influence on tides.

The figure below shows where the Moon is relative to the Sun at different times during the month (
Figure
below
). The positions of the Moon and Sun relative to each other affects the tides. This creates spring tides or neap tides.

Spring tides
occur during the new moon and full moon. The Sun and Moon must either be in a straight line on the same side of Earth, or they must be on opposite sides of Earth. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range.

Neap tides
occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range.